A heart is a hollow tissue formed of cells that are capable of producing a contraction that changes the length and shape of the cell. Heart pumps blood in cyclic contractions through a network of arteries and veins called the cardiovascular system. As shown in FIG. 1, a human heart includes four chambers, which are divided by a septum into a right side (right atrium RA and right ventricle RV) and a left side (left atrium LA and left ventricle LV). During a heartbeat cycle, the right atrium RA receives blood from the veins and pumps it into the right ventricle and the right ventricle RV pumps the blood into the lungs for oxygenation. The left atrium LA receives the oxygenated blood from the lungs and pumps it to the left ventricle LV, and the left ventricle LV pumps the blood into the veins. The apex AP of the heart is a portion formed by the inferolateral part of the left ventricle LV.
Various techniques have been developed to provide measurable parameters that are indicative of cardiac operation of a monitored subject. Many of these techniques are invasive and therefore suitable for advanced medical use only.
In the noninvasive side, echocardiography is a technique that applies ultrasound to provide an image of the heart. Echocardiography can be comfortably carried out at the bedside, and it has therefore become a widely-used tool for noninvasive studies on cardiac mechanics of diseased and healthy hearts. The produced images require, however, complex and basically immobile computer equipment and the images need to be interpreted by a highly trained physician. Ambulatory or long-term monitoring of the cardiac operation outside the clinical environment by echocardiography is practically impossible.
Electrocardiography is based on measuring electrical activity of the heart with electrodes attached to the surface of the skin of the monitored subject. In electrocardiography, wave depolarization of the heart is detected as changes of voltage between a pair of electrodes placed in specific positions on the skin. Typically a number of electrodes are used, and they are arranged in combination into pairs (leads). Electrocardiograms are very accurate and widely used, and also allow some computerized interpretation. Proper placement of the electrodes may, however, be challenging for users without medical training. In addition, the measurement system typically requires a computerized system connected with cables to a plurality of self-adhesive pads that couple through conducting gel to the skin of the monitored subject. Moving with such wiring is very limited.
Patent publication WO2010145009 discloses an apparatus for determining information indicative of physiological condition of a subject. The apparatus comprises a sensor device that obtains ballistocardiograph data indicative of heart motion of the subject, measured along a plurality of spatial axes. Ballistocardiograph data indicates the extent of mechanical movements of a body that take place in response to the myocardial activity of the heart. This ballistocardiograph data is then used to process data that is indicative of heart motion of the subject. This prior art method overcomes some of the limitations of the prior art. However, it has been noted that the linear measurement along spatial axes is strongly affected by the posture of the monitored subject during the measurement. In addition, some characteristics of the heartbeat cycle are not completely reliably measurable with the linear motion data.